Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Structures which are constructed of resin-fiber composite materials may be exposed to heat, which may affect the composite materials in various ways. These heat-induced effects may include chemical degradation in which changes such as oxidation, material loss and the breaking and/or forming of chemical bonds occurs in the polymer chemical structure of the composite materials. Resin decomposition, charring and fiber decomposition of the composite materials may occur at increasing temperatures.
Composite materials and epoxy-based surfacing films are typically cured under a vacuum at 250-350° F. for several hours. Repairs to the composite materials or surfacing films can become necessary if voids are found during an NDI inspection or if a flaw is found. These repairs may require the use of heating blankets that are notorious for having hot spots. Thus, during the repair some areas of the composite material or surfacing film may become overheated; in the case of epoxy-based surfacing films, a color change (dark or blackened) may be noticeable. Thermal effect may affect the mechanical and structural integrity of composite materials and surfacing films.
Repair or removal of heat-affected composite materials or surfacing films on a structure may involve first determining the degree of harmful thermal effect to the composite materials or surfacing films. Although determining the degree of thermal effect to composite materials or surfacing films may be performed by visual inspection, thermal effect may not be visually apparent. Current methods of determining the presence and extent of thermal effect in composite materials and surfacing films includes obtaining an infrared spectrum of a heat-affected composite standard and correlating the infrared spectrum obtained from the composite standard with the degree of thermal effect of the composite standard. An infrared spectrum obtained from the composite material or surfacing film the thermal effect of which is in question can then be compared to the infrared spectrum obtained from the composite standard to determine the presence and assess the degree of thermal effect in the composite material or surfacing film.
Calibration of infrared sensors to residual strength in composite materials or surfacing films correlates the resin condition as read from the infrared spectrum to the residual strength of the material or film which degrades as the resin degrades with progressively increasing temperatures. Therefore, the infrared sensors may be calibrated using time-controlled thermal soak standards which are obtained by exposing various composite materials or surfacing film controls to various temperatures for a particular time period such as one hour, for example. One method of preparing the standards includes placing the standards in an oven which is calibrated periodically and monitored continuously.
Near infrared radiation is capable of penetrating paint layers on a composite material or surfacing film to detect underlying composite material or film conditions. Hand-held near IR devices are generally less expensive than hand-held FT-IR devices and are often lighter and easier to use as well. Therefore, a method for accurately assessing thermal effect in a composite material or surfacing film using near-infrared spectroscopy is needed.